The present invention relates to photo-control devices and associated photo-control circuits for use in such devices.
Light-activated circuits (referred to herein as "photo-control circuits") are used in photo-control devices to control a wide variety of equipment including indoor and outdoor lighting, security systems, signs, as well as other kinds of electrically-operated appliances. A conventional photo-control circuit employs a photo-cell which responds to ambient light conditions.
Conventional photo-control circuits are limited by the ratings of their components. Photo-control circuits that are used in photo-control devices to drive smaller loads often employ components that can handle only small load currents, whereas photo-control circuits that are used in photo-control devices that drive large loads require scaling components or components with high power handling capacity. While both configurations provide light-activated control, each presents disadvantages. For instance, photo-control circuits designed to drive small loads have limited utility. Photo-control circuits that have high power capabilities, however, are not ideal substitutes as the required components increase cost. Another disadvantage of conventional photo-control circuits is reliability. When a photo-cell fails, the entire photo-control device fails.
FIGS. 1-3 depict a conventional type of photo-control device 10. FIG. 1 shows a front view, partially cut away, of the device 10. The device 10 includes a photo-control circuit 11 located in a housing 12. The housing 12 includes a translucent or transparent panel 14. Behind the panel 14 is located a single photo-cell 16 mounted on a mounting plate 18. The single photocell 16 comprises part of the conventional photo-control circuit 11. As shown in FIG. 2, behind the mounting plate 18 are located the remaining components 19 of the photo-control circuit 11. FIG. 3 shows a circuit diagram of the photo-control circuit 11 of the conventional photo-control device 10. The photo-control circuit 11 has an input 20 comprised of a line input 22 and a neutral 24. The conventional photo-control circuit 11 has an output 26 comprised of an output line 28 and a neutral 30. The input 20 connects to a power source 32 and the output 26 connects to a load 34. The single photo-cell 16 is connected in a first current path 36 between the line input 22 and the neutral 24. In this conventional device, the single photo-cell 16 has a 1 watt rating and a corresponding 1 inch diameter size. A heating resistor 38 is also in the first current path 36. The heating resistor 38 has a rating of 9.1 K Ohms. A switch 40 is connected in a second current path 39 between the input line 22 and the line output 28. The switch 40 is operated by a bimetal element 42 which in turn opens and closes the switch 40 relative to the heat generated by the heating resistor 38. When the photo-cell 16 is exposed to light, its resistance decreases which results in increased current flow through the branch 36 causing the heating resistor 38 to dissipate additional heat. The additional heat forces the bimetal element 42 to expand which moves the switch 40 into an open position disengaging the load 34 connected to the output terminal 28 from the source 32 connected to the input terminal 22. In the absence of significant light, the increased resistance of the photo-cell 16 limits the flow of current through the branch 36 which in turn limits the heat generated by the heating resistor 38. Without additional heat, the switch 40 returns to and remains in a normally closed position coupling the load 34 attached to the output terminal 28 to the source 32 coupled to the input terminal 22.
A problem that can be encountered with this type of circuit is that the photo-cell may fail. Under a normal failure mode, the photo-cell opens which in turn prevents the switch, e.g. 40, from disengaging from its normally closed position. While the use of premium photo-cells may improve reliability, these components have certain disadvantages. Premium photo-cells may not be readily available and may only have a limited operating range. Premium photo-cells sell at a high cost. Also, if a premium photo-cell fails, the control circuit will still also fail.
It is therefore an object of the invention to provide an improved photo-control device that improves reliability over the prior art.
Another object of the invention is to provide a photo-control device that is less expensive to manufacture.